The present invention relates to a light emitting diode (“LED”) lamp and an LED display unit, which are preferably used, for example, in outdoor LED display boards for displaying road information such as traffic information as well as for outdoor display for advertisement, signalers and the like.
Conventionally, there has been an LED lamp shown in FIG. 10. This LED lamp is structured such that an LED chip 102 mounted on a lead frame 101 is resin-molded and embedded in a resin lens 103. The resin lens 103 has a curved surface 103A that is curved in a convex shape in the light emitting direction of the LED chip 102 (see Japanese Patent Laid-Open Publication HEI 11-154766 and Japanese Patent Laid-Open Publication HEI 10-22529, for example).
The conventional LED lamp shown in FIG. 10 produces a phenomenon, when used in an outdoor LED display board for example, that in the evening and early morning when an irradiation angle of sunlight is low, sunlight directly comes incident upon the LED lamp and gives the unlit LED lamp the appearance as if it is lit. This phenomenon lessens a contrast ratio of a lit LED lamp to an unlit LED lamp, which may make the LED display board less recognizable. This phenomenon, which is similar to misread of signalers due to afternoon sunlight, causes remarkable deterioration of display quality of the LED display board that constitutes electronic billboards, signboards, and the like.
With reference to FIG. 12, more detailed description will be given of the above-stated phenomenon of the conventional LED lamp. When the rays of sunlight 111a, 111b with low irradiation angles in the evening and early morning, as shown in FIG. 12, become incident upon a resin lens 103, they are refracted by a curved surface 103A of the resin lens 103, so that their traveling directions approximate to normal lines L101, L102 at incident points P101, P102. By this refraction, the rays of sunlight 111a, 111b are incident upon and reflected by a portion of a lead frame 101 on or around which an LED chip 102 is mounted, as a consequence of which, at an incident angle close to the emission angle of the emitted rays of light from the LED chip 102, the reflected rays of light 111a, 111b comes incident upon and refracted by the curved surface 103A of the resin lens 103, and therefore the refracted rays of light exits in approximately parallel to an exit optical axis J101. This is the phenomenon that an unlit LED lamp looks like a lit LED lamp.
Next, another conventional example is shown in FIG. 11. This conventional example is similar to the above-described conventional example in that an LED chip 102 is mounted on a lead frame 101, but different in that there is provided a lens 105 having upper and lower curved surfaces 105A and 105B whose shapes are different from each other (see Japanese Patent Laid-Open Publication HEI 11-154766 for example). If the upper and lower curved surfaces are considerably different in curvature as with this conventional example, rays of light are concentrated upon an interface of these two curved lens surfaces 105A, 105B, which generates an irregular emission peak on the front side. Consequently, the apparent luminescence intensity becomes remarkably different between a view from the front side and a view with a viewpoint slightly displaced downward from the front side, thereby causing degradation of display quality.
Also, when the positional relationship between the light emitting diode chip and the interface of the lenses is changed due to production variations, the emission angle at the above-mentioned emission peak varies, so that particularly an LED display board composed of a lot of light emitting diode lamps disposed in a matrix form produces uneven display when viewed from the front side.
In the former conventional example as described above, the installation site of the LED display board should be taken into consideration. In the places exposed to afternoon sunlight, the board cannot be installed at all or should be installed with the display surface of the LED display board inclined downward. Therefore, particularly in the case of traffic information boards, there have been cases in which the conventional board could not be applied as a display board whose original function is to secure traffic safety.
Also, as is the case with the latter conventional example, simple combination of upper and lower lenses with different curvatures produces uneven display as viewed from the front-side direction.